The invention relates to pulse oximeters. More particularly, it relates to methods for reducing power consumption in pulse oximeters by selectively disabling circuitry used to monitor blood oxygenation.
Pulse oximetry refers to a process for determining the oxygenation level of the blood in a patient""s blood stream. It is particularly useful when monitoring persons with chronic pulmonary blockages and patient undergoing surgery.
One problem with pulse oximeters is their high power consumption. The front-ends of pulse oximeters use high power consumption devices such as LED""s and amplifiers to generate light and amplify that light after it passes through perfused tissue. Several times per second, pulse oximeters generate individual pulses of light that are transmitted through perfused tissue. These pulses are then received by a photodetector (typically a photodiode) and the amount of light of at least two different wavelengths is received. These individual pulses of light are then separately a serially processed to determine the degree to which the blood is perfused. The light intensity is digitized in analog to digital converters and is numerically manipulated to indicate the degree (typically the percentage) of oxygenation.
Blood oxygenation does not change instantaneously from pulse to pulse. Any change in oxygenation is a gradual process. Yet generating the pulses and calculating the corresponding oxygenation level happen virtually instantaneously. Thus, there is a significant amount of time in which pulses are not generated and processed. In typically oximeters, the circuitry that processes the pulses is powered up all the time, however. As a result, the circuits consume much more power than is needed. In the relatively long intervals between actual pulse generation and pulse measurement, the circuitry consumes power. This unnecessary power consumption can rapidly drain a battery powered oximeter and require the operator to either plug the oximeter into an AC supply that constantly trickle-charges the battery. This defeats the purpose of having a battery itself. Alternatively, the oximeter must be monitored regularly to insure it does not fail due to an exhausted battery. This defeats the purpose of having a monitoring device.
Not all components of a pulse oximeter can or should be shut down between pulse measurements, however. For example, the display that indicates the oxygenation levels should indicate the current oxygenation level at all times. Second, the microcomputer circuitry that keeps track of oxygenation trends should not be powered down, since this would typically reset its memory and cause it to xe2x80x9cforgetxe2x80x9d the previous oxygenation history.
It is an object of this invention to provide a method and apparatus for selectively de-powering the light generating and amplifying circuits of a pulse oximeter between actual pulse measurements. It is also an object of this invention to provide a method and apparatus for de-powering the light receiving and amplifying circuits of a pulse oximeter between actual pulse measurements.
In accordance with a first embodiment of the invention, a low power pulse oximeter is provided having a sensor further comprising a source of light configured to generate pulses of light in at least two wavelengths of light and a photodetector configured to receive the pulses of light and to generate an analog electrical signal indicative of the amplitude of light in each of the pulses, at least one amplifier that is coupled to the photodetector to amplify the analog electrical signal, a power supply that is coupled to the at least one amplifier to provide power to the at least one amplifier, an analog to digital converter that is coupled to the amplifier to receive and convert the amplified analog electrical signal into a digital signal, a microcomputer configured to receive the digital signal and to calculate a blood oxygenation level therefrom, and a switch disposed between the at least one amplifier and the power supply to selectively interrupt a flow of amplifier power from the power supply to the at least one amplifier.
In accordance with a second embodiment of the invention, a low power pulse oximeter for driving and monitoring an oximeter sensor is provided, wherein the sensor includes at least two LED""s configured to generate pulses of light at two different wavelengths and at least one photodetector responsive to the two wavelengths of light, the oximeter including a power supply configured to provide electrical power, a switch coupled to the power supply to selectively enable and disable a flow of the electrical power from the power supply between successive pulses of light thereby reducing power consumption, a preamplifier stage configured to be coupled to the at least one photodetector to receive an electrical signal from the photodetector representative of an amount of light impinging on the photodetector and to amplify that electrical signal, wherein the preamplifier stage is coupled to the switch to receive the flow of electrical power, an analog to digital converter stage coupled to the preamplifier stage to convert the amplified signal to a corresponding digital representation, and a microcomputer coupled to the analog-to-digital converter stage to receive the digital representation and calculate an oxygen saturation.
In accordance with a third embodiment of the invention, a method is provided for reducing the power consumption of a pulse oximeter having a photodetector for detecting pulsatile variations in light transmission through perfused tissue, a preamplifier stage to amplify electrical analogues of the pulsatile variations indicative of an oxygen perfusion, an analog to digital converter for converting an output of the preamplifier into a corresponding digital representation of the output, and a microprocessor configured to convert the digital representation to a value indicative of an oxygen perfusion, the method including the steps of: (a) generating a pulse of infrared light; (b) converting at least a portion of the pulse of infrared light into an infrared electrical signal; (c) energizing the preamplifier stage; (d) amplifying at least a portion of the infrared electrical signal in the preamplifier stage; (e) transmitting the amplified infrared signal to the analog to digital converter; (f) deenergizing the preamplifier stage; (g) generating a pulse of red light; (h) converting at least a portion of the pulse of red light into an red electrical signal; (i) reenergizing the preamplifier stage; (j) amplifying at least a portion of the infrared electrical signal in the preamplifier stage; (k) transmitting the amplified infrared signal to the analog to digital converter; and (l) deenergizing the preamplifier stage.
In accordance with a fourth embodiment of the invention, a low-power pulse oximeter is provided, including a microcomputer configured to generate a digital signal indicative of a desired amount of light, a digital to analog converter coupled to the microcomputer to convert the digital signal into an analog signal indicative of a desired amount of light, at least one amplifier coupled to the digital to analog converter to amplify the analog signal to an amplitude sufficient to generate the desired amount of light, a power supply coupled to the at least one amplifier to provide power to the at least one amplifier, a switch disposed between the power supply and the at least one amplifier to selectively interrupt a flow of amplifier power from the power supply to the at least one amplifier, and a light source coupled to the at least one amplifier and configured to generate the desired amount of light upon receipt of the amplified analog signal.
In accordance with a fifth embodiment of the invention, a medical monitoring system is provided, including a sensor configured to sense a physical parameter of a patient and to generate a sensor signal, at least one amplifier that is coupled to the sensor to amplify the sensor signal, a battery that is coupled to the at least one amplifier to provide power to the at least one amplifier, an analog to digital converter that is coupled to the amplifier to receive and convert the amplified signal to a digital signal, a microcomputer configured to receive the digital signal and to calculate a value indicative of the physical parameter therefrom, and a switching circuit disposed between the at least one amplifier and the battery to selectively interrupt a flow of electrical power from the battery to the at least one amplifier.